Cardiovascular morbidity and mortality is higher among Veterans than the general population independent of factors such as chronic illnesses or socio-econamic status. Depression is now recognized as a non-traditional risk factor for cardiovascular disease. Nearly one-third of Veterans suffer from depression at some point, regardless of whether they have been deployed in combat. Activation of vasopressin (AVP) receptors within the central nervous system, specifically the paraventricular nucleus (PVN) has been implicated in depression. Recent studies have shown that AVP is released from dendrites within the PVN and that central AVP mediates the sympathoexcitation observed heart failure. Sympathoexcitation is also strongly associated with greater cardiovascular risk. Notably, gamma-aminobutyric acid (GABA) typically suppresses sympathoexcitation. New evidence indicates that GABA may exert a paradoxically stimulatory effect on AVP signaling due to plasticity that occurs in the chloride ion concentration within neurons. The intracellular chloride concentration is controlled by chloride transport via the sodium chloride co-transporter 1 (NKCC1) which transports chloride ion into the cell or the potassium chloride co-transporter 2 (KCC2) which extrudes chloride ion. Thus, we hypothesize that AVP activates V1a receptors (V1aR) and/or V1b receptors (V1bR) within the PVN to increase arterial pressure, heart rate and sympathetic activity thereby contributing to the augmentation of these responses to acute stress in an animal model of depression. Three specific aims will be addressed. In Specific Aim 1, we will use pharmacologic inhibition and genetic knockdown with siRNA approaches to assess whether exogenous AVP activation of V1aR or V1bR alone or in combination results in increased arterial pressure, heart rate and RSNA and exaggerated responses to acute stress. In Specific Aim 2, we will test whether changes in NKCC1 or KCC2 transport in PVN attenuate, or even reverse, GABAergic inhibition of hemodynamic and RSNA responses to V1aR and/or V1bR activation in the basal state or during acute stress. In Specific Aim 3, we will ascertain whether endogenous AVP within the PVN activates V1aR and/or V1bR thereby contributing to the increased arterial pressure and RSNA in a rat model of chronic unpredictable stress (CMS), a validated model of depression, and whether increased transport via NKCC1 or decreased transport via KCC2 prevents GABAergic suppression of these responses. We will test this hypothesis in conscious, unrestrained Sprague Dawley rats chronically-instrumented with telemetry transmitters for both hemodynamic and nerve activity measurements. The ability to monitor not only arterial pressure but also RSNA by telemetry in conscious rats has been mastered by only a few laboratories including our own and provides a powerful tool for assessment of basal and stress conditions with minimal investigator interference. We will identify the contribution of the vasopressinergic receptor(s) involved in the responses to exogenous AVP as well as to acute stressors: air jet, nasopharyngeal reflex, tail in 50C water, and restraint stress. We will then ascertain whether GABA inhibition of AVP signaling is altered by blockade of NKCC1 or KCC2. Then, we will subject the rats to CMS and assess whether blockade of V1aR and/or V1bR decreases baseline arterial pressure, heart rate and RSNA. We will evaluate whether CMS predisposes to enhanced responses to acute stressors and if that response is due to vasopressinergic signaling that may be impacted by plasticity of the GABAergic system. With the exciting advent of new brain-permeant, highly selective V1aR antagonists already in phase I trials in other disorders, the need for studying V1aR and V1bR antagonism in stress disorders is timely and distinctly translatable for treatment of people with chronic stress such as depression. The proposed studies will provide the crucial rationale and robust mechanistic evidence upon which to design a clinical trial. Given that our Veterans experience both depression and a high risk for cardiovascular morbidity and mortality, the proposed pre-clinical studies will lay a vital foundation for new adjunctive treatments to improve outcomes for depressed Veterans.